1. Field of the Invention
The present invention relates to a fluid bearing device that utilizes dynamic pressure of a fluid, and a spindle motor having the fluid bearing device. The fluid bearing device of the present invention is applicable to fluid bearing devices that are particularly suitable for spindle motors of a hard disc drive and other disc drives, but is also applicable to other apparatuses.
2. Description of the Related Art
In a bearing device of a spindle motor used in a hard disc drive and the like, in place of a ball bearing device that has been conventionally used, a fluid bearing device that excels at rotating precision and that excels at sound silence than the ball bearing device is becoming widely used.
FIG. 11 is a cross sectional view of a main part of a conventional fluid bearing device disclosed in JP-A 2003-65323 (referred to as Patent Document 1) and JP-A 7-63220 (referred to as Patent Document 2). In the conventional fluid bearing device shown in FIG. 11, a bracket 53 made of a material that does not allow a lubricating oil serving as an operating fluid to pass through is provided at an outer periphery of a sleeve 52 configured by a sintered body and formed with a radial dynamic pressure generating groove 51 at an inner peripheral surface of a bearing hole. A shaft 54 is inserted in the bearing hole of the sleeve 52, and a radial fluid bearing is configured by the sleeve 52 and the shaft 54. A thrust plate 56 formed with a thrust dynamic pressure generating groove 55 is fixed at a bottom opening of the bracket 53. A thrust flange 57 is provided at a lower end of the shaft 54 so as to face the thrust plate 56, and a thrust fluid bearing is configured by the thrust flange 57 and the thrust plate 56. The lubricating oil serving as the operating fluid is filled in between the sleeve 52 and the shaft 54 and between the thrust flange 57 and the thrust plate 56, including a gap configuring the radial fluid bearing and the thrust fluid bearing.
In the configuration of the conventional fluid bearing device, the sleeve 52 is configured with a sintered body using a sintered metal material. The sintered body can form the radial dynamic pressure generating groove 51 by placing a sintered molded object in a metal mold and performing press molding. Thus, the radial dynamic pressure generating groove 51 can be formed at high precision with a simple step. That is, in the sleeve manufactured using a solid metal material that is not a sintered body, a groove must be formed by precise cut machining in the post-process. Thus compared to such, the sleeve 52 configured by the sintered body reduces the manufacturing cost as groove machining does not need to be performed in the post-process.
The sintered body has therein a great number of void pores. If the sleeve 52 is configured by the sintered body, the lubricating oil serving as the operating fluid may leak out to the outside through the void pores of the sleeve 52. As a result, the lubricating oil decreases and may affect the function as a fluid bearing. Further, the pressurized lubricating oil present in the radial dynamic pressure generating groove 51 may flow into the void pores of the sleeve 52 during the rotation of the motor, and the pressure generated by the dynamic pressure generating groove may be reduced thus lowering the rigidity of the bearing serving as the radial fluid bearing.
In order to solve the aforementioned problems, in the fluid bearing device of Patent Document 1, the sleeve 52 formed by the sintered body is surrounded by the bracket 35 formed with a material that does not pass the lubricating oil. The lubricating oil is thus prevented from leaking out to the outside through the void pores of the sleeve 52. In another conventional fluid bearing device disclosed in Patent Document 2, a glazing process is performed on the inner peripheral surface of the sleeve of the sintered body to block the void pores. This prevents the lubricating oil from leaking out from the radial dynamic pressure generating groove into the void pores of the sleeve during rotation, thus preventing the rigidity of the bearing of the radial fluid bearing from lowering.
JP-A 2003-333792 (referred to as Patent Document 3) discloses a configuration in which a shaft is formed by a sintered body where a sintered metal material is sintered and a lubricating oil is contained in the shaft of the sintered body. A sleeve to which the shaft is inserted is configured with a metal solid that does not penetrate the lubricating oil, and the shaft is supported in the sleeve so as to be lubricated and rotated by the lubricating oil. The bearing is referred not as a hydrodynamic bearing but as a cylindrical slide bearing. In this configuration, the shaft is formed by the sintered body instead of the sleeve. However, since the sleeve is formed with a material that does not penetrate the lubricating oil, the lubricating oil contained in the shaft does not leak out from the sleeve.
In the conventional fluid bearing device disclosed in Patent Document 1 and Patent Document 2, however, a bracket formed with a material that does not pass the lubricating oil must be disposed on the outer sides of the sleeve formed with the sintered body. Thus, the number of components increases, so that the number of manufacturing assembly processes increases and, also, the manufacturing cost increases. Further, in miniaturizing the hard disc drive, as has been recently desired, two components of the sleeve and the bracket are necessary and thus becomes difficult to miniaturize.
Further, in the conventional fluid bearing device disclosed in Patent Document 3, the cylindrical slide bearing is simply configured, and thus the rigidity of the bearing lowers as the porosity of a porous sintered body configuring a rotating shaft increases, and cannot respond to the lowering of the bearing rigidity.